1. Field of the Invention
The present invention relates to an image reading device which is used in such as a digital copy machine or an image scanner, and an image forming device including the image reading device.
2. Description of the Related Art
In recent years, the development of the light emitting diode (LED) has been carried out actively, and brightness of LED elements has risen rapidly. Generally, advantages of LED are, for example, long life, high efficiency, high anti-acceleration character and monochrome luminescence, etc., and numerous applications of LED in the field of illumination are expected.
As an application, LED is used as a manuscript illuminating device of an image reading device such as a digital copy machine or an image scanner.
Various methods are proposed for the application of LED which is employed by the image reading device. For example, JP 2006-67551A, JP 2006-42016A, JP 2005-241681A disclose that a plurality of LEDs are lined up in parallel with a main-scanning direction of a manuscript, and light emitted from the plurality of LEDs is diffused without giving any optical effect in the main-scanning direction, and the light is collected by reflecting surfaces in a sub-scanning direction, thus illuminance on a reading target area on a manuscript surface is heightened. Further, JP 2005-311662A discloses that a plurality of LEDs are lined up in parallel with a main-scanning direction of a manuscript, and light emitted from the plurality of LEDs is diffused without giving any optical effect in the main-scanning direction, and the light is collected by a lens in a sub-scanning direction, thus illuminance on a reading target area on a manuscript surface is heightened.
Here, the desired kind of illuminance or illuminance distribution on the manuscript surface will be explained.
As illustrated in FIG. 1, in an image reading device used in a digital copy machine or an image scanner, information included in a manuscript is input into an imaging device such as CCD through a readout lens. FIG. 2 is a top view of a manuscript surface illustrated in FIG. 1 viewed from the above. The image reading device is in a state such that only information of a long and thin reading target area illustrated in FIG. 2 is capable of being input into the imaging device, when an optical system is in a fixed state. Then, by moving the entire device illustrated in FIG. 1, or moving an illumination optical system in conjunction with reflecting mirrors, the reading target area is moved in a direction of an arrow in FIG. 2. The entire manuscript can be read out by inputting information into the imaging device sequentially while moving the reading target area.
At this time, the amount of light entering the imaging device per unit time is required to be increased to move the reading target area illustrated in FIG. 2 at a high speed (to shorten reading time per one sheet of the manuscript), and a high illuminance to the manuscript surface is preferable. From this viewpoint, the methods of restricting light in the sub-scanning direction disclosed in the above-mentioned patent documents are appropriate.
Meanwhile, uniform illuminance distribution is preferable in general. FIG. 3 illustrates a conjugate relationship of the manuscript surface and the imaging device, and an example of the illuminance distribution on the manuscript surface is illustrated by a solid line and a dotted line. The solid line in the figure represents an illuminance distribution on the manuscript surface at a predetermined point, and the dotted line represents a changed illuminance distribution by such as an influence of a vibration from outside at a predetermined moment. As illustrated by the solid line or the dotted line in FIG. 3, when there is unevenness in the illuminance distribution on the manuscript surface, illuminance at a position on an imaging surface corresponding to a position on the manuscript surface where the illuminance is high, is high as well, and vice versa. During moving the reading target area in the direction of the arrow illustrated in FIG. 2 and reading the entire manuscript, if a relationship of the manuscript and the illumination is always the same, illuminance distribution with such unevenness can be corrected by an image data processing. However, unevenness in density is generated in a scanned image if the relationship of the manuscript and the illumination is changed from a state illustrated by the solid line to a state illustrated by the dotted line instantaneously during this period, and a quality of the image decreases.
Therefore, as illustrated in FIG. 4, in general, it is preferable that the illuminance distribution over the entire reading target area be uniform. Illuminance distribution on the imaging surface would not change even if a positional relationship of the manuscript and the illumination shifts, for example, by receiving a vibration from the outside, if the illuminance distribution is uniform.
In any method disclosed in the above-mentioned documents, to make the illuminance distribution in the main-scanning direction uniform, light from a point light source, namely an LED, is diffused directly without giving any optical action to the light. However, when such a method is adopted, a long distance is necessary from the light source to the manuscript surface when the number of light source is small, and there is a possibility that an illumination optical system from the light source to the manuscript surface gets larger. When increasing the number of light sources to miniaturize the illumination optical system, though the illuminance is improved and the illumination optical system can be further miniaturized, there are disadvantages such as a high cost and high power consumption.